using LAFS as a key-value store

[Zooko Wilcox-OHearn]

I uploaded a slightly edited version of this to github so that you can
see the rendered version of it with hyperlinks:

https://github.com/zooko/tahoe-lafs/blob/3c13c138cf09e83d2f8001888e2a7de85564d406/docs/frontends/key-value-store.rst

There's also a pull-request to add it to the docs directory:

https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2229

Regards,

Zooko